Conventionally, it has become easy to completely separate elements by utilizing the SOI (Silicon On Insulator) structure, and it has become possible to suppress soft errors and latch-up peculiar to CMOS. Further, it has become evident that, if a surface Si layer of SOI is thinned to about 100 nm and the impurity concentration is also controlled to relatively low so that the substantially entire Si layer of SOI is depleted, then further excellent effects such as suppression of short channel effect and increase of current driving capability of MOSFET can be obtained. As a technique for evenly forming a thin film SOI layer on the order of 100 nm over a large area, a SIMOX (Separation by IMplantation OXygen) method has very vigorously been studied. In this method, oxygen ions are implanted deep into the Si substrate at a high concentration, and Si and the oxygen are made to react with each other in the subsequent heat treatment to form a buried SiO.sub.2 layer in the Si substrate leaving a thin monocrystalline Si layer on the surface thereof. The amount of oxygen implanted is selected to be about 10.sup.18 cm.sup.-2 and an acceleration voltage of ion implantation is selected to be 150 to 200 KeV. After implantation, a high heat treatment is conducted for about 4 to 6 hours at the temperature of about 1300 degrees Centigrade.
However, if a semiconductor integrated circuit of high speed operation and low power consumption in which the MOS transistor and the bipolar transistor are hybridized on the SOI substrate, then the following problems occur. Hereinafter such a semiconductor integrated circuit is abbreviated as BiCMOS circuit.
In the MOS transistor adopting the SOI structure, it is necessary to make the Si layer thin in order to completely deplete a channel forming area in the operating condition and to prevent a kink characteristic. Typically, the Si layer is formed having a thickness of below 100 nm. In the BiCMOS circuit, it is difficult to fabricate a vertical type npn bipolar transistor at the Si thin film layer of SOI structure. That is, there is a limit posed for securing withstand voltage or maintaining the breakdown voltage sufficiently on the reduction of sizes of the n-type emitter area, p-type base area and n-type collector area taken in the longitudinal direction (vertical direction), and if, in particular, the n.sup.+ -type buried layer is formed, it is difficult to secure the base-collector and emitter-collector withstand voltages or maintaining the breakdown voltage sufficiently. On the other hand, unless the n.sup.+ -buried layer is formed, the resistance of the collector layer becomes high with the result that the saturation of the collector current becomes easy to occur and the cut-off frequency f.sub.T is lowered.
Further, there is also a method of utilizing a horizontal type npn bipolar transistor which is easy to combine with the MOS transistor of SOI structure rather than the vertical type bipolar transistor, but the former bipolar transistor is easily influenced by the effect of the substrate surface, and is difficult to obtain a high current gain and a high breakdown voltage.
In order to solve the problems which occur when the BiCMOS circuit is formed on the SOI substrate, a method of selectively providing a bulk region on the SOI substrate to form the bipolar transistor there is described in S. Matsumoto et al, ELECTRONICS LETTERS, 6th Jul., 1989, vol. 29, No. 14, pp. 904-905. However, in this method, when a locaI SIMOX substrate leaving the bulk region locally is manufactured according to the oxygen ion implantation process, defects often occur at the boundary between the SOI portion and the substrate portion, and the yield of the devices is likely to be reduced.
One of the methods of preventing these defects is described in Japanese Patent Application Laid-Open No. 61-121468. That is, as shown in FIGS. 1A through 1E, a resist 505 is provided at at least the principal part of a portion 500 which serves as the SOI structure on the substrate 501, a trench 504 is dug, and further, an deep oxygen ion implantation is effected selectively to the portion 500 which serves as the SOI structure by utilizing a mask 505'. It is further heat treated to change an,oxygen ion implantation area 502' into a stable buried oxide film 502 to form a semiconductor substrate having a partial SOI structure.
Incidentally, in FIGS. 1A to 1E, 506 denotes an oxide film, 507 an epitaxial layer, 508 an incomplete epitaxial layer and 509 an oxide film.
However, it was found! out that the foregoing method involves the following drawbacks. That is, unless the depth of the trench 504 is formed deeper by more than a predetermined value than the depth of the buried oxide film 502, even if the foregoing method is used, defects will occur.
Further, in order to evenly form a thin Si layer of about 100 nm within the substrate surface, it is necessary to prevent channeling and, to this end, the substrate is disposed within the ion implantation apparatus so that the oxygen ion beam comes incident at an angle of 14 degrees relative to the normal line of the substrate 501. Therefore, if, as shown in FIG. 2A, after the trench 504 is previously formed within the semiconductor substrate 501, oxygen ions are implanted (as indicated by arrow 100), an area 510 shadowed by a mask 505' as ions are implanted is formed, and oxygen ions cannot be evenly introduced into the area surrounded by the trench. Since the Shadowed portion is deficient in the amount of oxygen implanted, an excellent buried oxide film cannot be formed. For example, if the amount becomes lower than (0.3-0.4).times.10.sup.-18 cm.sup.-2, then voids are formed and an evenly buried oxide film cannot be obtained. On the other hand, if the amount of oxygen implanted is increased inorder to sufficiently introduce the oxygen into the shadowed portion, then the amount at the unshadowed portion becomes too much, and after a high temperature annealing, irregularity occur on the surface of the substrate. If, for example, the amount of oxygen implanted becomes above 2.5.times.10.sup.18 cm.sup.-2, then the SOI surface will become rough.
Incidentally, there are methods in which, as shown in FIG. 2B, ions are implanted with an ion implantation mask 505' having an inclined edge to make the shadowed area 510 of the mask small, or as shown in FIG. 2C, ions are implanted with an ion implantation mask 505' having an edge displaced from the inner side of the trench 504. But either of them involves a complicated process, and since there is the trench when the ions are implanted, the shadowed portion 510 cannot be completely eliminated. Therefore, a method of forming the local buried oxide film simply and evenly has been needed.